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8 July 2013 Last updated at 11:51 ET By Jonathan Amos Science correspondent, BBC News

The lower slopes of Mount Sharp can be seen at top-left. The northern rim of Gale Crater is at top-right

Nasa's Curiosity Mars rover has finally begun the long drive to its primary mission destination - Mount Sharp.

For the past seven months, it has been investigating a site just east of its August 2012 touchdown point, drilling rocks and analysing their composition.

But scientists have now decided it is time for Curiosity to get rolling.

On Friday, engineers commanded the vehicle to make an 18m drive. On Monday it travelled 40m. It will however take many months to reach Mount Sharp.

The rover has to steer clear of a long bank of sand dunes that represent a potential trap. The intention also is to get to a specific site where satellites have indicated there are layers of sediment that were potentially laid down in water.

All up, this could see Curiosity having to roll roughly 8km to get to the places of key scientific interest. And if the cameras onboard spot unusual rocks, the rover will be commanded to park and examine them.

The mission team announced in early June that its extended stay at the small depression known as Yellowknife Bay was almost over.

Although a diversion to the main mission, it proved to be a very productive one. Curiosity found mudstones that were very probably deposited billions of years ago in a lake.

The rover - also called the Mars Science Laboratory (MSL) - was put down in Mars' equatorial Gale Crater on 6 August last year.

Its mission goal is to probe the crater's sediments for evidence that past environments on Red Planet could once have favoured microbial life.

The rover has spent recent months examining rocks that owe their origin to the action water

Jonathan.Amos-INTERNET@bbc.co.uk and follow me on Twitter: @BBCAmos